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Bone Densitometry What is the Fundamental Basis - Tissue
Interaction?
There are essentially two
methods to measure the density of bone (BMD). We may pass either X-rays or ultrasound waves through the bone being assessed, and measure the
effect the bone has on these waves.
X-ray: X-rays are invisible waves similar to radio or
television waves. When they pass through our bones, some of the X-rays are absorbed by
calcium atoms in the bone. Measuring how many X-rays are absorbed indicates the bone
density.
QUS: Quantitative Ultrasound (QUS) is a high frequency sound wave.
We may measure how quickly sound travels through bone, this is termed velocity which is
measured as metres per second (m s-1), or how much sound is absorbed by the bone,
generally referred to as Broadband
Ultrasound Attenuation (BUA, dB MHz-1).
Density:
Volumetric or Areal
As mentioned previously, bone mineral
density (BMD) may be measured within a volume of bone (g cm-3), termed volumetric density, or within a known area of bone (g cm-2), termed areal density.
What Densitometry Methods are there and Where do
they Measure?
Radiogrammetry
This is based upon conventional X-ray
techniques, effectively producing a see-through photograph of our bones. By taking an
X-ray of the spine or hand, we may determine if any of our spinal vertebrae are shortening or whether the
bone in our fingers is thinning.
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hand X-ray |
SXA and DXA
Absorptiometry
This is the most popular technique,
incorporating either one (single energy X-ray absorptiometry, SXA) or two (dual energy
X-ray absorptiometry, DXA or DEXA) X-ray energies.
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DXA System |
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DXA bone
densitometry records the areal BMD (g cm-2), and is routinely measured at the lumbar spine (lower portion
of the spine), hip and forearm (wrist).
For the lumbar spine, the vertebrae L1 to L4 or L2 to L4 are routinely measured. Above L1,
the ribs may appear in the scan area, although this effect may be minimised by performing
a lateral scan. For the
hip, several regions may be studied including the femoral neck, trochanter and Ward’s triangle. DXA may
also be used to measure the composition of our total body - the amount of lean tissue and
fat.
A recent development
has been Morphometric X-ray
Absorptiometry (MXA), where the size
and shape of the vertebrae may be recorded in order to determine if any have suffered compression fractures or have become wedge
shaped.
SXA may also be utilised,
particularly for assessment of the forearm and calcaneus (heel).
Again, areal density is recorded.
QCT
Quantitated Computed Tomography, often termed a CAT Scan, may
also be utilised to measure the bone density of the spine. The potential advantage of QCT
is that it measures volumetric rather than areal density. Also, the bone density of cancellous bone
alone may be measured, separate from the cortical shell, by selecting a region of
interest (ROI).
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QCT machine |
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QCT image |
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QCT may also be used
to measure volumetric BMD of the forearm, often termed peripheral (limb)
QCT (pQCT).
QUS
Due to technical difficulties,
Quantitative Ultrasound measurements cannot routinely be performed at the common
anatomical sites affected by osteoporosis (spine, hip and wrist). However, it has been
clinically demonstrated that ultrasound measurement of the calcaneus (heel) provides an accurate indication of osteoporosis
fracture risk, particularly for hip fracture. Other anatomical sites that are routinely
measured by ultrasound include the phalanges (fingers) and tibia (shin).
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CUBA
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How are Densitometry Results Interpreted?
Once a BMD measurement has been
performed, it has to be clinically interpreted. The most common way of doing this is to
adopt the WHO definition for
Osteoporosis, based upon the BMD T-Score.
How do we calculate T- Score?
The BMD T-Score is a measure of how a subject's BMD value compares to those of a typical young normal subject, defined in terms of the standard deviation of young normal subjects:
T-Score = subject’s
BMD value - mean young normal BMD value
young normal BMD standard deviation
Definitions:
mean =
average BMD value of young normal subjects
standard deviation = statistical
spread of population BMD values around the mean value
It should be noted that
the T-Score may also be calculated for other bone densitometry parameters such as
ultrasound velocity and BUA.
The WHO define a subject as being Normal at the measured site if their T-Score is above
-1.0. Similarly, a subject is defined as being Osteopenic (moderate osteoporosis) at the
measured site if their T-Score
is between -1.0 and -2.5; and
Osteoporotic at the measured site if their T-Score is -2.5 or below.
The WHO also define Severe (or Established) Osteoporosis if a subject's BMD is defined as being osteoporotic and they also have
suffered one or more fragility fractures.
Alternative Parameters
Z-Score
The Z-Score compares a subject’s BMD value with those of the similarly
aged (age-matched) population, defined as the number of standard deviations (SD) below age-matched
subjects:
Z-Score = subject’s
BMD value - mean age-matched BMD value
age-matched BMD standard deviation
Percentage Expected
The Percentage Expected describes a subject's BMD
value as a percentage of the average BMD value from a similar aged population, defined as:
Percentage Expected = subjects'
BMD value x 100
mean age-matched BMD
Problems with applying the
WHO Criteria
When the WHO criteria were
established in 1994, they were primarily intended to investigate the number (prevalence) of subjects suffering from osteoporosis in different countries of the
World, rather than as a clinical tool to diagnose osteoporosis in an individual.
The definitions were based
upon early forearm BMD data, and assumed that the T-Score threshold values could also be
implemented for other BMD measurements, for example, the lumbar spine and hip. We now know
that different parts of the skeleton behave differently within any particular subject;
they may lose differing amounts of bone at the spine, hip and heel; and hence may have
different BMD T-Score values at different measurement sites.
A direct consequence of this
is that a particular subject may be, for example, 'normal' at their
forearm, 'osteopenic' at their hip and 'osteoporotic' at their
lumbar spine. This is one of the reasons why clinical consultation
is an important factor in the diagnosis and management of osteoporosis – ‘over the counter’ results can be misleading without
sufficient explanation.
It is widely agreed,
however, that the WHO definitions for normal, osteopenic and osteoporotic results may be
applied to DXA measurements of the lumbar spine, hip and forearm. However, the WHO
definitions should not be applied to the calcaneus (heel). Considerable effort is
currently being made world-wide to provide a means of comparing all bone densitometry
results, from all techniques and all commercial systems, for all measurement sites, in
both male and female subjects of all races.
What are the Potential
Artefacts for BMD Measurements?
Vertebral Compression
Fractures
If a subject suffers a compression fracture (shortening) of one or more of their vertebrae,
then the same amount of bone mass will be contained within a reduced volume - this will
effectively increase the measured BMD value. Hence,
even though the subject will have suffered an osteoporotic fracture, their BMD will appear
to have 'improved'.
Degeneration
As the body ages, arthritic changes may
develop, the edges of the vertebrae becoming ‘lumpy’ with an associated increase
in measured BMD.
Calcification
Another factor associated with ageing
is that calcium may become deposited on the linings of the arteries. This calcium may
absorb some of the X-rays, again giving the false appearance of an increased BMD.
What is the Variability between and within Systems?
For a given bone
densitometry technique such as DXA, different manufacturers' systems perform the same
fundamental measurement but often adopt slightly different techniques. This means that
small variations may occur if a subject was measured on different manufacturer's systems.
A smaller variation would occur if the subject was measured on different systems from the
same manufacturer. An even smaller variation would occur if the subject was measured on
the same system, often referred to as repeatability or precision, where we would
expect the variability to be in the region of 1%. Since bone loss at the lumbar spine
associated with the menopause is typically 2-3% per year, reducing to approximately 1%
five years after the menopause, we tend to repeat DXA scans every one or two years. If we
performed them more often, we would not know for certain whether any decrease or increase
in BMD was due to real changes in the skeleton, or the inherent variability of the
measurement itself.
Safety Issues
Radiation Exposure
DXA uses X-rays, however the radiation
dose is very small. It has been compared to the exposure during a transatlantic flight.
Ultrasound Safety
Ultrasound is a mechanical wave with no
known biological effect at the intensities used clinically. All of the present QUS
machines have intensity below the maximum levels imposed by the US Food and Drug
Administration.
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